An endoscope or wand viewing apparatus is usually configured in a way that is well suited for illumination and imaging in visible light.
In this background section endoscopes and wand like devices shall be described in terms of their internal working components. In the second portion of the background section the shortcomings of these predicate devices shall be described with respect to their use for laser induced fluorescence imaging.
An endoscope is an opto-mechanical imaging device characterized by the following: an objective lens assembly containing an optional deviating prism assembly, a relay system forming a plurality of intermediate images for the case of a rigid endoscope, an ocular, a camera system containing multiple detectors with a prism assembly directing sections of the electromagnetic spectrum to dedicated sensors, or in the case of chip-on-a-stick endoscope where there are no intermediate images; an objective lens assembly coupled directly to the camera detector which is embedded in the distal most portion of the device. In either case the optical elements are contained in an inner most tube member which is surrounded by other tube members which may include fiber optics for illumination in the annulus formed by two or more of the tubes and may contain other tubes for the passage of instruments and or irrigation.
An objective lens assembly in the distal most portion of the device forms a real image of the scene coincident with the plane of the detector in the case of a chip-on-a-stick endoscope or intraoral camera, or distal most plane of a relay system that is contained within the shaft portion of the endoscope.
In an endoscope with a relay system (the second case above), there are 2 basic forms of relays, those of lenses and those using coherent imaging fibers.
In both examples the role of the relay is to reform the image produced by the objective lens through the length of the shaft by producing intermediate images in the case of a rigid endoscope to a new position where the ocular, in the case of a visual endoscope or camera lens group in the case of a digital or electronic imaging device, may then reform the image originally produced by the objective lens group for the eye or to a camera detector.
The shaft portions of all endoscopes are made to facilitate the insertion of the device into a body cavity or body lumen, that is to say diameter is the dimension being minimized. In the case of a body cavity insertion, the shaft is often rigid and comprised of thin walled stainless steel tubes. This tube within tube construction allows for an innermost tube to contain the optical train and then surrounding tubes can contain fiber optics to transmit illumination to the scene in the form of an annulus. Additional tubes can be contained within the assembly for the introduction of surgical instruments for various purposes.
For the case of an endoscope that utilizes coherent imaging fibers for the relay system the functional concept being optimized in the device is flexibility, and to some degree what is being compromised is resolution, particularly when the diameter of the tip is small. However, large diameter flexible endoscopes often use detectors directly behind the objective lens assembly and are therefore considered “chip-on-a-stick” configurations. These flexible endoscopes often have internal channels, instrument and irrigation channels, to pass forceps, etc. and have internal guide wires and steering mechanisms at the tip controlled by levers at the proximal end of the device.
There is a class of smaller diameter endoscopes utilizing coherent imaging fibers as relays whose shafts have a limited amount of flexibility, and these devices are commonly called semi-flexible. Often configured with a working channel, called a forceps channel, used for instruments and are often used in Urology.
Whether rigid, flexible, or semi-flexible, endoscopes have a proximal eyepiece section for viewing and or coupling to a camera system. An eyepiece is not present on chip-on-a-stick endoscopes or intraoral cameras, often called dental cameras, as the camera is imbedded into the distal portion of the device.
Where eyepieces are used, manufacturers have almost universally adopted a nominal 32 mm eye cup for blocking room light from the physician's view, and this eye cup serves to support the coupling mechanism to the camera. There are some commercial applications of directly coupling the camera and optical assemblies included in the shaft mechanism and or coupling mechanism but this has not found wide acceptance, except in Orthopedics for Arthroscopy. The fear among users has been that if the electronics of the camera fail for some reason then the doctor is left with no means to view within the patient, hence the continuing presence of endoscopes with eyepieces.
The class of endoscopes not utilizing eyepieces is commonly called chip-on-a-stick. The intraoral dental camera shares the lack of eyepiece or ocular, as well. Both instruments send a video signal to a monitor or computer for viewing.
Endoscopes are most commonly fixed focus imaging devices. There is a broad distance from the tip of the device to the subject that is in focus due to the relatively small aperture of the optical system. A small aperture allows only a small amount of light to be imagined for any point in the scene. Should focusing be required it is accomplished by repositioning the optics in the camera module proximal to the endoscope, or in some cases the detector itself is moved.
Such low levels of return signal in the visible spectrum require endoscopes to have large light sources such as xenon, halogen, and metal halide.
In the case of an endoscope, commonly called a chip-on-a-stick which contain a distal most detector (CCD, CMOS, or other sensor), the change in image plane position for a near object of interest versus a far object of interest is usually ameliorated by installing a very small aperture in the objective lens assembly to increase depth of field at the expense of a bright field or higher potential resolution.
This is a distinction between endoscopes and chip-on-a-stick endoscopes regarding focus. Endoscopes with proximal cameras do provide a means for focus even if they themselves are fixed focus. Chip-on-a-stick endoscopes usually do not provide a focus means. However, large diameter flexible endoscopes used in gastroenterology do often have moveable detectors or lenses providing focus.
For smaller chip-on-a-stick endoscopes, all of the optical elements in the objective lens assembly are optimized for the small aperture which allows great depth of field. It is not the case that the aperture could be removed for increased brightness. The advantage is that no motion (focus) is required and when provided with powerful illumination systems in the visible light the overall system can perform well given the constraint of illumination.
A focus means becomes an area of distinction between a chip-on-a-stick endoscope and a wand imaging device, referred to as an intraoral or dental camera, as well. Both chip-on-a-stick endoscope and a wand imaging devices contain the detector plane in the distal or forward portion of the device directly behind the objective lens assembly. The intraoral or dental camera is often required to be used in stand off mode, a distance that is greater than endoscopy requires. The distinction results from the use; endoscopy is done in closed surgical or diagnostic sites, the intraoral devices are used external to the body or inserted in natural cavities such as the mouth. In such stand off modes, not the mouth but full face views, great amounts of illumination would be required to satisfy the large change in s and s′ (the optical path length on either side of the objective lens) in the intraoral or dental camera where to be fixed focus. Therefore, intraoral or wand like dental cameras frequently contain a means to move the detector plane, or focus the device. Using a faster F number, with inherently less depth of field but higher sensitivity, and by moving the detector plane a lower powered illumination system is required. This allows a wand with near IR capabilities to accommodate large changes in distances to the object of interest, thereby allowing a faster optical system to be designed, a characteristic that requires variable focus, but provides higher inherent resolution, and more conservative illumination sources.
Endoscopes and wands are generally designed to visualize in the visible spectrum. However, fluorescent dyes, such as indocyanine green (ICG) (Akorn, Inc., Buffalo Grove, Ill.) are commonly being used to image anatomy in the infra red spectrum. Use of ICG is described in, for example, U.S. Pat. No. 6,915,154, which is incorporated herein by reference in its entirety. Once excited, ICG emits in the infra red spectrum at about 825 to about 835 nm. FIG. 1 shows the excitation and emission spectrum of the ICG composition sold by Akorn, Inc. There is therefore a need for endoscope and wand devices that are capable of imaging and visualizing in the infrared spectrum.